Comment 70 for bug 662840

The default for a site you haven't visited is to match the language of the document or the edit control. If you change the dictionaries manually, they should be remembered for the site, so this should only be a problem the first time you visit a site.

I think it's worth discussing whether we should change this behaviour when the user has explicitly enabled multiple dictionaries at the same time, but I think that would best be done either in a new bug or maybe one of the existing bugs above.